linux-kernel - Re: Kernel marker has no performance impact on ia64.

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Date:	Thu, 12 Jun 2008 15:34:46 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Hideo AOKI <haoki@...hat.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel marker has no performance impact on ia64.

Hi Frank,

Frank Ch. Eigler wrote:
> Hi -
> 
> On Thu, Jun 12, 2008 at 01:05:52PM -0400, Masami Hiramatsu wrote:
>> [...]
>>>> "sched_switch(struct task_struct * next, struct task_struct * prev)":"next %p prev %p"
>>>> out of tree. Thus, you can use the printf-style format parser.
>>> That's an interesting idea, but errors in this table would themselves
>>> only be caught at C compilation time.
> 
>> Hmm, why would you think so?  I think if we can't find corresponding
>> entry from the lookup table, it becomes an error.
> 
> Sure, but if the entry exists but is wrong, we'd emit C code that
> won't compile.

I think if someone changes the trace point in the kernel,
Module.markers is also changed.

ex.)
 DEFINE_TRACE(sched_switch, (struct task_struct * next, struct task_struct * prev),
	     next, prev);

if someone changes above to below,

 DEFINE_TRACE(sched_switch, (int prev_pid, int next_pid), prev_pid, next_pid);

Module.markers also change like

 sched_switch	vmlinux	(struct task_struct * next, struct task_struct * prev)

to

 sched_switch	vmlinux	(int prev_pid, int next_pid)

In this case, the below entry never matches to new Module.markers.

"sched_switch(struct task_struct * next, struct task_struct * prev)":"next %p prev %p"

Thus, we can find an error.
However, of cause, we should take care of the task_struct changes.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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